1. Field of the Invention
This invention pertains to the field of ultrasonic imaging for medical diagnostic purposes. Systems for such diagnostic imaging include an ultrasonic transducer, imaging electronics, and display apparatus. The imaging electronics actuate the transducer for propagating incident ultrasonic energy into a patient's body. Within the patient's body, the ultrasonic energy causes echoes at interfaces between body tissues having differing acoustical impedance characteristics. Some of these echoes are reflected back to the transducer, which converts them to electrical output signals. The imaging electronics process the electrical output signals to cause the display apparatus to produce visual images representing structure of the patient.
One ultrasound system is known as a real time B scanner whose transducer is located in a chamber or vessel containing a water bath. Drive apparatus moves the transducer along a path within the water bath, simultaneously with the production of bursts of ultrasonic energy by the transducer. The drive system includes monitoring equipment which produces signals indicating transducer positions along the path.
In use, an operator positions the water bath chamber over a region of interest of a patient. The drive apparatus moves the transducer and the imaging electronics initiate the ultrasonic energy production. The electrical signals processed by the transducer in response to the echoes from these incident pulses, and the transducer position indicating signals, are processed to produce a rapid series of visual images of the patient. Such images are typically produced at a rate of approximately 15 to 30 per second, enabling the visualization of motion of, or changes in, internal components and organs of the subject's body.
A discussion of various ultrasonic techniques, including the known B scan method, is found in an article entitled "Methods of Acoustic Visualization", by Green, P. S. et al, International Journal of Nondestructive Testing, Vol. I (1969) pp. 1-27. Another example of such a system is described in United States patent application Ser. No. 725,178, filed Sept. 21, 1976, now U.S. Pat. No. 4,141,347, by Green, et al entitled REAL TIME ULTRASONIC B-SCAN IMAGING AND DOPPLER PROFILE DISPLAY SYSTEM AND METHOD.
2. Description of the Prior Art
Mechanical drive systems are known for moving transducers in the described real time B scan systems. U.S. Pat. No. 4,065,976 issued Jan. 3, 1978 to Taenzer, et al discloses one proposed system which has been used clinically in a research atmosphere but which has not been made commercially. With this proposal, transducer mounting structure forming an articulated parallelogram is used. The parallelogram structure includes two relatively long side legs, and two relatively shorter end legs which are pivotally connected together. An ultrasonic transducer is mounted on one of the shorter legs. The transducer and the articulated parallelogram structure are immersed for motion in the water bath.
The end leg of the parallelogram structure opposite the transducer is supported within the water bath. In operation, drive structure, powered by an electric motor, and coupled to the parallelogram structure, articulates the structure. This motion causes the transducer to move along a curved path. During motion, the transducer is actuated by electrical signals transmitted over leads extending from the imaging electronics outside the water bath and through the walls of the fluid bath chamber.
While the drive system described above has exhibited utility it has several attendant disadvantages. These disadvantages stem from (1) the size, mass and complexity of the system; (2) the excessive agitation of the bath during system operation; (3) mechanical balancing problems; (4) water sealing difficulties associated with multiple members extending through the fluid chamber walls, (5) nonlinearity of the relation between the transducer position and the drive structure operation, and (6) the large number of moving parts which result in unneeded complexity and cost.
Another problem with a parallelogram system is that its multiplicity of parts all exhibit hydrodynamic drag when rapidly moved in water. This phenomenon produces stresses upon the moving parts and their connecting structures, and requires considerable power to maintain their movement at the rapid speeds desired, and with the required precision for imaging. The additional power requirement further increases the size, weight and complexity, as well as the cost, of the drive system.
The motion of all these parts also undesirably agitates the water bath. Moreover, the fact that the curved transducer motion path has components extending in more than one direction complicates this problem. As a result of water agitation images may be blurred and the life expectancy of components, such as bearings, is unduly limited.
In addition, the parallelogram system is inherently difficult to balance mechanically, and this difficulty in balancing leads to the presence of undue vibration in the scanner unit when in operation. The vibration problem is aggravated by the relatively high moving mass of the system.
Another drawback of this prior system is that it requires multiple elements penetrating the walls of the water bath chamber. Each penetration requires an associated sealing structure, which increases the cost and complexity of the system. Moreover, the seals, particularly those associated with movable parts, tend to fail after extensive use.